Field of the Invention
The invention relates to a method for producing complex formed castings of an AlCu alloy.
Description of Related Art
When information about contents of alloy elements is provided here, said information respectively relates to the weight of the relevant alloy, unless expressly indicated otherwise.
Castings consisting of AlCu alloys of the type concerned here have particularly high strengths, especially at elevated working temperatures of more than 250° C. However, this is accompanied by poor casting characteristics which complicate the casting-production of components, which are characterized by a complex shaping.
Typical examples of such castings are cylinder heads intended for internal combustion engines which, on the one hand, are exposed to high temperatures during practical use and on the other hand have a compact construction form in which filigree-shaped form elements, such as cooling channels and oil channels, recesses, webs, guides and the like are formed.
A fundamental problem of processing substantially Si-free AlCu alloys lies in their high susceptibility to cracking under heat and in a backfeed behaviour which is much poorer than in the case of conventional AlSi alloys.
WO 2008/072972 A1 discloses a method for producing complex formed castings of an AlCu alloy which consists of (in % by weight) 2-8% Cu, 0.2-0.6% Mn, 0.07-0.3% Zr, up to 0.25% Fe, up to 0.3% Si, 0.05-0.2% Ti, up to 0.04% V and as remainder Al and unavoidable impurities, wherein the total of the contents of impurities not amounting to more than 0.1%. Particular importance is given to the presence of Zr with regard to the production of a fine structure, with grain sizes of at most 100 μm.
To improve the fineness of the casting structure, during implementation of the known method of a respectively composed melt, before casting a grain refiner such as TiC can additionally be added in a dosage of typically 2 kg per ton of melt. The casting which is obtained after casting and solidification undergoes a heat treatment in which it is initially solution annealed at 530-545° C. The casting is cooled down in an accelerated manner from the solution annealing temperature using water or in an air stream, wherein quenching with water in particular is considered as advantageous in respect of the desired high strength, but cooling in an air stream is recommended in case the casting tends to form cracks during a relatively fast cooling procedure due to its complex shaping. After the quenching, the casting is kept at a temperature of 160-240° C. for a duration of 3-14 hours to increase the hardness of the structure.
Attempts at practically implementing the known method have shown that the known alloy does admittedly have advantages regarding the material characteristics which make it particularly interesting for the casting-production of cylinder heads for internal combustion engines. However, using the known method, it is not possible to produce castings of this alloy with the required operational reliability on a large scale which meet the demands imposed on them during practical use.
Thus, it has been found that depending on the cast, the grain size of the respectively obtained castings does in fact extremely vary. Thus for example an average grain size of approximately 100 μm could be observed on a very large sample piece which solidified very slowly. However, when a smaller piece is separated from this sample, when it is again melted and is then allowed to solidify again very rapidly, then in spite of the fast solidification rate, against expectations grain sizes of 500-900 μm are found. Castings having such a coarse structure are completely unsatisfactory for the use intended by the methods concerned here.